This invention relates to medical devices useful for performing laser angioplasty, a specific type of laser surgery designed for the treatment of atherosclerosis and the like.
Atherosclerosis is a disease that causes the thickening and the hardening of the artery walls. It is characterized by lesions of raised atherosclerotic plaque which form within arterial lumens and occlude them partially or wholly. Coronary atherosclerosis tends to increase progressively in humans with age, and is a leading cause of death in the United States. The treatment of atherosclerosis typically includes drug therapy, surgery and/or percutaneous transluminal coronary angioplasty (PTCA).
Progress in interventional cardiovascular techniques has led to increasing reliance on non-surgical coronary and peripheral revascularization such as balloon angioplasty. In keeping with that trend, a complex array of new technologies has been developed that seek to improve on the results of balloon angioplasty. While balloon angioplasty has been a great advancement in treating selected patients with coronary and peripheral vascular disease, it has the possible undesirable effects of causing severe dissection, abrupt closure, and restenosis, such that the chronic result may be worse than the pre-treatment lesion. Moreover, even after ten years of improvements in technique and catheter design, these complications and the degree of their incidence have not changed significantly, albeit, more complex patients are being treated. Indeed, models of experimental atherosclerosis have taught us that endothelial denudation, as produced by balloon abrasion, initiates the proliferative intimal response, allows lipid deposition, and accelerates the atherosclerotic process.
Some of the newer technologies in clinical trials seek to improve the luminal geometry of stenotic lesions aiming at less traumatic restructuring by intravascular stenting, plaque removal by atherectomy or sealing of intimal dissections and decreasing elastic recoil by laser balloon angioplasty. Stents, however, are subject to foreign body reaction and require initial balloon dilation with resultant endothelial injury. The popular atherectomy devices cause mechanical trauma as well as balloon injury from the apposition balloon, and laser balloon angioplasty results in both thermal and balloon injury. While all these procedures have potential clinical applications as adjuncts to balloon angioplasty in the next several years, there is clearly a need for a nontraumatic technique of lesion ablation to carry us into the next era of effective and precise endovascular surgery.
To date the most promising technique suitable for precise tissue removal is that of laser angioplasty. Clinical systems currently in use are primitive at best, particularly those employing conventional lasers for bare fiber or "hot tip" laser angioplasty resulting in largely undirected thermal destruction. Continuous wave conventional lasers, such as the Nd:YAG laser emitting radiation in the near infrared region (1064 nm) and argon lasers emitting light in the visible spectrum, have little utility for efficient tissue removal. At these wavelengths, tissue absorption is diminished resulting in a thermal rather than an ablative process. Part of this problem is due to the absorption spectrum of water, the major constituent of vascular tissue. In contradistinction, peaks of the absorption spectrum in the ultraviolet region around 300 nm, and in infrared region such as at 2900 nm, suggest that lasers such as the XeCl excimer and the erbium YAG lasers, respectively, may be excellent plaque ablators. In fact, clinical data support this finding, with evidence of precise tissue ablation with either laser. The other laser characteristic affecting the degree of the thermal versus the ablative response is the mode of laser delivery, either continuous wave or pulsed. While continuous wave lasers normally lead to deep thermal penetration with possible charring and shallow craters, control of pulse duration and repetition rates can maximize the ablative properties of pulsed lasers as well as positively affect the particle size of ejected tissue. In fact, even pulse lasers can achieve impressive tissue ablation with minimal thermal effects.
Two main technical problems have stood in the way of effective utilization of these lasers clinically. The first is the difficulty of transmitting high power pulsed lasers in conventional fiberoptics. Recently, however, the excimer laser has been transmitted down a 200 micron fiberoptic using an expanded pulse technique, and prototype fiberoptics show considerable promise for transmitting erbium YAG laser radiation. It is therefore likely that fiberoptic capability will not be the limiting factor for development of a successful laser angioplasty system using these lasers. The second and more substantial problem that prevents clinical utility of an ablative laser is that of providing safe, reliable and precise laser guidance to change an intravascular weapon to a precision instrument. To date most laser ablative systems employ a forward firing laser beam with or without a metal cap and constructed with or without a lens systems to convert laser energy into purely thermal energy. These types of systems are all capable of opening channels to various degrees, with some ulility in the case of total occlusions, by allowing passage of a guide wire for subsequent balloon angioplasty. However, none of these systems can achieve a normal luminal diameter as sole therapy. Angiographic guidance of these laser catheters has not been completely successful and is fraught with the hazard of vascular perforation, particularly in tortuous segments. While the peripheral arterial vasculature is large, straight, and relatively forgiving in the case of perforation, the coronary circulation shares none of these characteristics, and perforation is therefore intolerable. Alterations in tip design, while somewhat helpful in improving trackability, cannot lead to precise or reliable guidance. One known guidance method is the use of a proximal centering balloon for a laser angioplasty system. While this device has some utility in straight arteries for crossing shorter occlusions wherein the arterial segment proximal to the occlusion is free of disease and therefore results in appropriate centering, these conditions are infrequently met, and also require the use of balloon angioplasty with its known limitations.
A second known system for use in ablative laser angioplasty employs tracking of the laser catheter over a conventional guidewire. While the use of guidewire offers the only reasonably safe method of using an ablating laser at the present time, the inability to determine precise depths of laser penetration continues to result in ineffective luminal improvement and often requires subsequent balloon angioplasty.
Interest in creating a laser ablative system that distinguishes between normal and atherosclerotic tissue has led to the development of the concept of a spectroscopic guidance laser angioplasty system. This so-called "smart" laser is based on the premise that the induced fluorescence of atherosclerotic plaque provides an altered spectroscopic pattern, which, in turn, signals a computer generated algorithm to allow further laser pulses until a normal pattern is recognized. This system, however, requires direct apposition of a fiberoptic with the plaque, making intravascular application unfeasible without further guidance to direct the laser to circumferential lesions. In practice, however, the difference between normal and abnormal tissue is not always evident fron the spectroscopic pattern and atheroslerotic tissue can be full thickness, obviously allowing the potential for perforation.
Intraluminal sonography, recently miniaturized for percutaneous intravascular use, is one tool capable of circumferential imaging. Current technology allows imaging catheters as small as 0.8 mm with either phased array or rotating mechanical transducers, and either central channel or monorail systems for guidewire use. Catheters are now able to image vessels to 2 mm diameter and will likely be miniaturized further, making this a feasible imaging modality for coronary use.
Intraluminal sonography demonstrates distinct layers of the arterial wall. The intima and adventitia are echogenic, unlike the healthy media which is hypoechoic. Atherosclerotic plaques are echogenic and become hyperechoic when calcified, at times to the point of causing shadowing behind the structure. Vascular imaging has been shown to be extremely accurate in camparison to histologic sections of in-vitro human carotid and iliac vessels. Intraluminal sonography has been studied in dogs and pigs to image iliac, cerebral, mesenteric and coronary arteries. It has been documented to clearly define plaque removal by atherectomy as well. Therefore, as other imaging modalities fail to give as precise structural detail, intraluminal ultrasound appears to be a superior technology for guiding intravascular procedures such as laser angioplasty.